1. Field of the Invention
This invention relates to the field of radio-frequency identification systems (RFID) and in particular concerns improvements in RFID proximity readers.
2. State of the Prior Art
Radio-frequency identification (RFID) systems include passive systems in which the ID tag circuits are powered by energy radiated by the reader, and active tags which carry a battery. Passive ID systems tend to have a shorter operating range because actuation of the ID tags requires a sufficient RF field strength from the reader. Typical passive proximity readers have a relatively short range of operation, about two feet or less between reader and ID tag. Operating range is often an important criterion in the selection of a proximity system, and it is generally desirable to detect ID tags at somewhat greater range, such as five feet away, for greater flexibility in positioning of the reader unit or to cover a wider area with a single reader.
The reader in a passive ID system has an RF frequency generator and a tuned antenna circuit which establish a radio frequency field near the reader unit. The passive ID tags lack a frequency generator and instead communicate with the reader by loading down the reader's radiated field in a pattern which is detected by the reader and decoded as ID tag data. The ID tag includes a transponder integrated circuit which is powered by energy derived from the reader's RF field. The energy required by the tag transponder is received by means of a tag antenna circuit tuned to peak resonance at the reader's transmission frequency. The effective operating range of the reader/ID tag system is determined in part by the efficiency with which the RF energy is radiated by the reader and received by the tag. This calls for accurate tuning of the resonant antenna circuits of both tag and reader. In practice, however, environmental factors and manufacturing tolerances result in a departure from this ideal.
In existing passive proximity identification systems both reader and identification tags are intended to operate on a single common radio frequency, typically 125 kHz. Metallic and dielectric materials in the vicinity of the reader, and changes in temperature and humidity may all affect the tuning of the antenna circuits in the reader, resulting in less than optimal radiation of the RF field and reduced power transfer from the reader to the tags. That is, for I.D. tags of given sensitivity, the tag must be brought closer to the reader before the tag transponder will be activated by the reader's weakened transmission. The tuned antenna circuit of the tags is similarly affected by environmental factors and also by manufacturing tolerances, both of which may degrade tag performance by shifting the peak resonance of the tag away from the reader's operating frequency. If the tag tuning is off frequency, the operating range of the proximity system is again reduced because of diminished tag sensitivity and reduced loading of the reader's RF field. Tag performance can be improved by using high precision components, but such precision is costly. Five percent tolerance parts are much cheaper than one percent components, and ten percent tolerance parts are cheaper still. Since ID tags are often used in large numbers, it is desirable to keep the unit cost of the tags as low as possible.
In order to accommodate off-frequency drift in both the proximity reader and the ID tags, current practice is to use low-Q antenna circuits in the reader. Low-Q resonant tank circuits have a broader frequency response but at the expense of lowered sensitivity at the center frequency of the antenna circuits. The broader response allows the reader to detect off-frequency tags but at a lower level of system performance, that is, with lower sensitivity so that the tags must be closer to the reader before being detected.
A continuing need exists for more effective proximity readers capable of detecting passive ID tags at greater range and in particular more reliably detecting off-frequency ID tags.